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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="fusion.support.category_of"></a><a class="link" href="category_of.html" title="category_of">category_of</a>
</h3></div></div></div>
<h5>
<a name="fusion.support.category_of.h0"></a>
        <span class="phrase"><a name="fusion.support.category_of.description"></a></span><a class="link" href="category_of.html#fusion.support.category_of.description">Description</a>
      </h5>
<p>
        A metafunction that establishes the conceptual classification of a particular
        <a class="link" href="../sequence.html" title="Sequence">Sequence</a> or <a class="link" href="../iterator.html" title="Iterator">Iterator</a>
        (see <a class="link" href="../iterator/concepts.html" title="Concepts">Iterator Concepts</a> and
        <a class="link" href="../sequence/concepts.html" title="Concepts">Sequence Concepts</a>).
      </p>
<h5>
<a name="fusion.support.category_of.h1"></a>
        <span class="phrase"><a name="fusion.support.category_of.synopsis"></a></span><a class="link" href="category_of.html#fusion.support.category_of.synopsis">Synopsis</a>
      </h5>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">traits</span>
<span class="special">{</span>
    <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
    <span class="keyword">struct</span> <span class="identifier">category_of</span>
    <span class="special">{</span>
        <span class="keyword">typedef</span> <span class="emphasis"><em>unspecified</em></span> <span class="identifier">type</span><span class="special">;</span>
    <span class="special">};</span>
<span class="special">}</span>
</pre>
<h5>
<a name="fusion.support.category_of.h2"></a>
        <span class="phrase"><a name="fusion.support.category_of.parameters"></a></span><a class="link" href="category_of.html#fusion.support.category_of.parameters">Parameters</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Parameter
                </p>
              </th>
<th>
                <p>
                  Requirement
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody><tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">T</span></code>
                </p>
              </td>
<td>
                <p>
                  Any type
                </p>
              </td>
<td>
                <p>
                  The type to query.
                </p>
              </td>
</tr></tbody>
</table></div>
<h5>
<a name="fusion.support.category_of.h3"></a>
        <span class="phrase"><a name="fusion.support.category_of.expression_semantics"></a></span><a class="link" href="category_of.html#fusion.support.category_of.expression_semantics">Expression
        Semantics</a>
      </h5>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">category_of</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">category</span><span class="special">;</span>
</pre>
<p>
        <span class="bold"><strong>Return type</strong></span>:
      </p>
<p>
        The return type is derived from one of:
      </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">fusion</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">incrementable_traversal_tag</span> <span class="special">{};</span>

    <span class="keyword">struct</span> <span class="identifier">single_pass_traversal_tag</span>
        <span class="special">:</span> <span class="identifier">incrementable_traversal_tag</span> <span class="special">{};</span>

    <span class="keyword">struct</span> <span class="identifier">forward_traversal_tag</span>
        <span class="special">:</span> <span class="identifier">single_pass_traversal_tag</span> <span class="special">{};</span>

    <span class="keyword">struct</span> <span class="identifier">bidirectional_traversal_tag</span>
        <span class="special">:</span> <span class="identifier">forward_traversal_tag</span> <span class="special">{};</span>

    <span class="keyword">struct</span> <span class="identifier">random_access_traversal_tag</span>
        <span class="special">:</span> <span class="identifier">bidirectional_traversal_tag</span> <span class="special">{};</span>
<span class="special">}}</span>
</pre>
<p>
        And optionally from:
      </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">fusion</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">associative_tag</span> <span class="special">{};</span>

    <span class="keyword">struct</span> <span class="identifier">unbounded_tag</span> <span class="special">{};</span>
<span class="special">}}</span>
</pre>
<p>
        <span class="bold"><strong>Semantics</strong></span>: Establishes the conceptual classification
        of a particular <a class="link" href="../sequence.html" title="Sequence">Sequence</a> or <a class="link" href="../iterator.html" title="Iterator">Iterator</a>.
      </p>
<h5>
<a name="fusion.support.category_of.h4"></a>
        <span class="phrase"><a name="fusion.support.category_of.header"></a></span><a class="link" href="category_of.html#fusion.support.category_of.header">Header</a>
      </h5>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">support</span><span class="special">/</span><span class="identifier">category_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">category_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<h5>
<a name="fusion.support.category_of.h5"></a>
        <span class="phrase"><a name="fusion.support.category_of.example"></a></span><a class="link" href="category_of.html#fusion.support.category_of.example">Example</a>
      </h5>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_base_of</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">category_of</span><span class="special">&lt;</span><a class="link" href="../container/list.html" title="list"><code class="computeroutput"><span class="identifier">list</span></code></a><span class="special">&lt;&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">list_category</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">traits</span><span class="special">::</span><span class="identifier">category_of</span><span class="special">&lt;</span><a class="link" href="../container/vector.html" title="vector"><code class="computeroutput"><span class="identifier">vector</span></code></a><span class="special">&lt;&gt;</span> <span class="special">&gt;::</span><span class="identifier">type</span> <span class="identifier">vector_category</span><span class="special">;</span>
<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">is_base_of</span><span class="special">&lt;</span><span class="identifier">forward_traversal_tag</span><span class="special">,</span> <span class="identifier">list_category</span><span class="special">&gt;</span> <span class="special">));</span>
<span class="identifier">BOOST_MPL_ASSERT</span><span class="special">((</span> <span class="identifier">is_base_of</span><span class="special">&lt;</span><span class="identifier">random_access_traversal_tag</span><span class="special">,</span> <span class="identifier">vector_category</span><span class="special">&gt;</span> <span class="special">));</span>
</pre>
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      Dan Marsden, Tobias Schwinger<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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